Composite materials can be obtained by superposing sheets each of which is constituted by juxtaposing lengths of resin-impregnated roving, each length of roving comprising a large number of individual carbon fibers, and then by hardening the impregnating resin. The diameter of the fibers is about 10 microns.
When such a composite material is subjected to shear stresses, loss of cohesion is observed between the laminations beyond a certain stress value, i.e. the various layers of the material separate from one another. The higher the shear stress at which the laminations lose cohesion, and consequently the better the carbon fibers adhere to the resin, the better the mechanical properties of the material. However, excessive adhesion leads to a fragile material, i.e. a material lacking in impact strength.
Proposals have already been made to improve the adhesion of the fibers to the resin in which they are to be embedded by surface treating raw carbon fibers as fabricated either by chemical means or by electrochemical means. Chemical groups are thus produced on the surface of the fibers which encourage adhesion to resins mostly by creating fiber-to-matrix chemical bonds, but also to some extent by increasing Van der Waals interactions between the fiber and resin components.
Electrochemical treatments of this type are described, for example, in French patent application No. 2 477 593. They consist essentially in immersing the fibers in an electrolyte solution and in polarizing them positively relative to a cathode. Good adherence is obtained in particular by using sulfates and bisulfates of ammonia and sodium as the electrolyte, i.e. strong saline electrolytes.
These electrolytes include oxygenated anions and result in oxygenated groups being grafted onto the carbon fibers. These oxygenated groups improve adherence to synthetic resins, but the method of treatment may, under certain circumstances, lead to the mechanical properties of the carbon fibers being degraded.
This French Application also mentions tests performed using strong acids and bases as the electrolyte (sulfuric acid, phosphoric acid, sodium hydroxide). As a result it is observed that either the hardening of the impregnating resin in which the fibers are later incorporated is inhibited, or else the treated fiber's resistance to oxidation is poor.